Polymers having an anionic group in a molecule are effective as functional polymers such as ion exchange resins and solid acid catalysts. Of these, polymers having a sulfonic acid group are particularly widely used because they have excellent properties due to the high dissociability of sulfonic acid group. Of these, sulfonated products of polyether sulfone (sulfonated polyether sulfone) and derivatives thereof are effective materials for forming thin films used in a variety of separation processes such as reverse osmosis, ultrafiltration, and ion exchange. Sulfonated polymers such as sulfonated polyether sulfone and sulfonated polysulfone have recently been reported to be effective as solid polymer electrolytes for fuel cells. A method for efficiently producing such sulfonated products has been desired.
Known examples of the method for producing sulfonated polyether sulfone (hereinafter sometimes referred to as “S-PES”) include a method for performing sulfonation in a chlorinated hydrocarbon solvent such as 1,2-dichloroethane, methylene chloride, and chloroform (for example, see PTL 1 and PTL 2), and a method for performing sulfonation using sulfuric acid as a solvent (for example, see PTL 3 and PTL 4).
Unfortunately, the method using chlorinated hydrocarbon as a solvent is difficult to industrialize because the molecular weight of a resulting S-PES is reduced, and a reaction product adheres to the walls of a reactor and mixing blades, making it difficult to remove the reaction product from the reactor. Chlorinated hydrocarbon has other problems such as severe toxicity and strict environmental regulations. The method using sulfuric acid as a solvent has a problem regarding significant reduction in the molecular weight of the resulting S-PES.